An impact energy absorber absorbs the impact energy generated from an external impact load caused by the collision of a vehicle. Therefore, conventionally, the impact energy absorber has been used to protect a passenger inside the vehicle. The impact energy absorber is set, for example, inside a door panel or a ceiling panel.
The impact energy absorbers having been conventionally employed as the structure that receives the impact load are roughly classified into a lattice shaped rib type, a truncated cone shaped rib type, and a long-groove rib type.
Patent Literature 1 discloses the long-groove rib type. This type includes a first wall disposed on a side which receives the impact, and a second wall separated from and opposed to the first wall across a hollow portion. Each of the first wall and the second wall has a long-groove shaped recess. Thus, the above type includes a plurality of impact absorbing ribs each having: a deep-groove portion having a welding surface formed therein by joining the front end surfaces of the recesses to each other; and, a shallow-groove portion having the front end surfaces of the recesses, the surfaces being separated from and opposed to each other. The long groove is formed by sucking one surface of each of two molten thermoplastic resin sheets toward the corresponding mold or by pressing the other surface thereof toward the mold. Next, the split mold blocks are clamped to weld the end surfaces of the long grooves together and form the hollow portion through the formation of an annular parting line.
Then, it has been known that the so-called double-wall impact energy absorber disclosed in Patent Literature 1 can absorb the highest impact energy by making the thicknesses of the ribs and of the grooves more even.
It has generally been known that the energy that absorbs impact is calculated by the product of the load and the area defined by a load-compressive strain curve and an axis representing compressive strain. The area defined by the load-compressive strain curve and the axis representing compressive strain is desired to be maximized. Therefore, in a region where the compressive strain is approximately 20 to 70% as indicated by a dotted line in FIG. 18, the waveform representing the substantially constant load close to the upper-limit value (hereinafter this waveform is also referred to as “rectangular waveform”) is ideal. Note that the upper-limit value is properly determined depending on the portion to which the impact absorber is attached and on the purpose of attaching the impact absorber.
It has been known that the impact energy absorber, disclosed in the above-mentioned Patent Literature 1, having the double wall molded to form a hollow by the blow molding can absorb the highest impact energy by making the thicknesses of the ribs and of the grooves more even. This has achieved a load-compressive strain curve extremely close to the ideal rectangular waveform that can be achieved only by the ideal impact energy absorber.